Heating and cooling systems such as heat pumps, air conditioners and refrigeration systems normally include an evaporator having an inlet line for receiving a liquid refrigerant from a condenser and an outlet line, or suction line, for carrying the vaporized refrigerant to a compressor. As refrigerant passes through the evaporator, it is converted by heat absorbed from the surroundings from liquid form to a vapor. Devices utilized to meter flow of refrigerant through the inlet line into the evaporator include the thermostatic expansion valve, the short tube orifice and the capillary tube. The thermostatic expansion valve includes a flow control valve that is opened or closed by a diaphragm, and a thermostatic bulb connected to the valve by a capillary tube. The thermostatic bulb and the interconnecting tube contain a thermally sensitive charge. Many types of charges are used in thermostatic expansion valve bulbs. Examples of charges include liquid and liquid cross-charges, as and gas cross-charges and adsorption charges. When the thermostatic bulb is heated or cooled, the pressure of the charge acts on the diaphragm and opens or closes the valve. Further details regarding thermostatic expansion valves are provided in the ASHRAE 1988 Equipment Handbook, pages 19.2-19.8.
In a conventional system, the valve portion of the thermostatic expansion valve is located in the inlet line to the evaporator and the thermostatic bulb is in thermal contact with the suction line, so that the flow of refrigerant into the evaporator is controlled in response to the temperature of the refrigerant vapor in the suction line. Typically, for low pressure drop evaporators, the vapor in the suction line is several degrees warmer than the liquid refrigerant entering the evaporator through the inlet line. The term "superheat" means raising the temperature of the refrigerant vapor above the temperature required to change the refrigerant from a liquid to a vapor at a specified pressure level. For low pressure drop evaporators, the superheat is approximately equal to the difference in temperature between the vapor in the suction line and the refrigerant in the inlet line. Typically, a superheat on the order of 8.degree.-20.degree. F. is required for proper operation of a thermostatic expansion valve. If the superheat drops below a prescribed value, indicating that the refrigerant is not being fully evaporated, the thermostatic expansion valve reduces the flow of refrigerant into the evaporator until the superheat returns to the prescribed value. Conversely, when the superheat exceeds the prescribed value, indicating that the refrigerant vapor is being overheated, the thermostatic expansion valve increases the flow of refrigerant into the evaporator.
Various problems have been associated with systems wherein a thermostatic expansion valve is used to control the flow of refrigerant into an evaporator. The superheat required for operation of the thermostatic expansion valve is a source of inefficiency. In order to provide the prescribed value of superheat, a portion of the evaporator near the suction line contains refrigerant vapor rather than liquid refrigerant. This portion of the evaporator operates less efficiently than the portion which contains a liquid refrigerant, since the heat transfer coefficient to a vapor is lower than to a liquid. Ideally, the entire evaporator should contain liquid refrigerant, and the refrigerant leaving the evaporator through the suction line should be fully vaporized. Liquid refrigerant passing through the suction line can potentially damage the compressor. Therefore, in an optimized system, the superheat should be reduced as much as possible without permitting liquid refrigerant to reach the compressor.
A further problem associated with thermostatic expansion valves is known as "hunting," which results from the time delay inherent in the control system. When the thermostatic expansion valve changes the rate of refrigerant flow, there is a time delay before the refrigerant is evaporated and causes a change in the sensed superheat. As a result, the system oscillates between a superheat above the desired value and a superheat below the desired value. This results in operating inefficiencies and inaccurate temperature control, and can potentially permit flow of liquid refrigerant to the compressor.
The prior art contains various proposals for dealing with the above-described problems and other problems associated with thermostatic expansion valves. A thermal electric expansion valve is disclosed by Wirgau in "Development of A Thermal Electric Expansion Valve," Appliance Engineer, Aug. 1984, pp. 52-55. The valve is electrically controlled by a thermistor positioned on the evaporator suction line. Other electrically-controlled expansion valves are disclosed by Miller in "Electronic Expansion Valve Offers More Precise Control In A/C, Refrigeration Systems," Air Conditioning/Heating and Refrigeration News, Dec. 2, 1984, and in U.S. Pat. No. 4,651,535 issued Mar. 24, 1987 to Alsenz. A solenoid flow control valve is controlled by a pulsewidth modulated control signal in which the duty cycle determines the flow rate through the valve. The Miller article describes microprocessor control of the solenoid control valve. In such configurations, the thermostatic bulb is eliminated. While such configurations have certain advantages, they have not found widespread use.
In U.S. Pat. No. 4,467,613 issued Aug. 28, 1984 to Behr et al, the superheat setting of a thermostatic expansion valve is automatically adjusted by an electric heater which biases the thermostatic bulb in response to a refrigeration parameter such as compressor lubricant temperature. While such configuration can reduce the superheat associated with the evaporator, significant energy is required to heat the thermostatic bulb with a resistance heater. U.S. Pat. No. 3,638,446 issued Feb. 1, 1972 to Palmer and U.S. Pat. No. 2,807,151 issued Sept. 24, 1957 to Baker also disclose cooling systems wherein the thermostatic bulb of a thermostatic expansion valve is heated with a resistance heater.
U.S. Pat. No. 4,441,329 issued Apr. 10, 1984 to Dawley discloses a refrigerator temperature control system including computer-controlled thermoelectric modules for heating or cooling temperature sensors during a sensor integrity test cycle. U.S. Pat. No. 3,237,415 issued Mar. 1, 1966 to Newton discloses a zone-controlled refrigeration system wherein thermoelectric units are utilized in each zone to directly control temperature.
It is a general object of the present invention to provide improved heat pump, air conditioning and refrigeration systems.
It is another object of the present invention to provide improved apparatus for controlling a thermostatic expansion valve.
It is a further object of the present invention to provide improved apparatus for controlling the flow of refrigerant to an evaporator in a heating or cooling system.
It is a further object of the present invention to increase the energy efficiency of heat pumps, air conditioners and refrigeration systems.
It is a further object of the present invention to provide apparatus for controlling a thermostatic expansion valve wherein hunting is eliminated.
It is yet another object of the present invention to provide control apparatus capable of heating and cooling the thermostatic bulb of a thermostatic expansion valve at different times.
It is another object of the present invention to provide apparatus including a thermoelectric heat pump device for controlling a thermostatic expansion valve.
It is another object of the present invention to provide apparatus for controlling a thermostatic expansion valve, which is easily adaptable for use in a variety of heat pump, air conditioning and refrigeration systems.
It is a further object of the present invention to provide apparatus for controlling a thermostatic expansion valve, which is low in cost and easy to manufacture.
It is a further object of the present invention to provide apparatus for controlling a thermostatic expansion valve having the capability to rapidly close the valve by active cooling of the thermostatic bulb.
It is a further object of the present invention to provide apparatus for controlling a thermostatic expansion valve having the capability to rapidly open the valve by active heating of the thermostatic bulb.
It is yet another object of the present invention to provide apparatus for controlling a thermostatic expansion valve having a combination of the above features.